This invention relates to a color plasma display panel for use in an information display terminal or a flat panel television and, in particular, to a color plasma display panel which is high in contrast and excellent in color fidelity or color reproducibility.
A color plasma display panel (hereinafter abbreviated to a color PDP) is a display in which ultraviolet rays are produced by gas discharge to excite phosphors so that visible lights are emitted therefrom to perform a display operation. Depending upon a discharge mode, the color PDP is classified into an AC (alternating current) or a DC (direct current) type. The AC type is superior to the DC type in luminance, luminous efficiency, and lifetime.
Referring to FIGS. 1 through 3, a conventional reflection AC type surface discharge color PDP will be described.
As illustrated in the figures, the conventional color PDP comprises a transparent glass plate as a front substrate 1. The front substrate 1 is provided with a plurality of transparent electrodes 2 arranged in stripes. In FIG. 2, the transparent electrodes 2 extend in a direction perpendicular to the drawing sheet. Between adjacent ones of the transparent electrodes 2, an AC pulse voltage of several tens to several hundreds kilohertz (kHz) is applied to cause discharge which triggers a display operation.
In the reflection AC type surface discharge color PDP, it is required to avoid interception of the visible lights emitted from phosphor layers 9R, 9G, and 9B which will later be described. To this end, the transparent electrodes 2 typically comprise a transparent conductive film of tin oxide (SnO2) or indium tin oxide (ITO) deposited by a thin film technique such as sputtering.
However, the transparent conductive film mentioned above is high in sheet resistance. In case of a large panel or a high-definition panel, an electrode resistance will become as high as several tens kiloohms (k.OMEGA.) or more. This may result in insufficient pulse rise or voltage drop of the pulse voltage applied to the transparent electrodes 2. In this event, it is difficult to drive the color PDP. Taking the above into account, it is proposed to provide each of the transparent electrodes 2 with a bus electrode 3 comprising a multilayer thin film of chromium/copper/chromium, a metal thin film such as an aluminum thin film, or a metal thick film using a silver paste. A combination of each transparent electrode 2 and each bus electrode 3 forms a surface discharge electrode set 2H reduced in resistance by presence of the bus electrode 3.
On the surface discharge electrode sets 2H, color filter layers 4R, 4G, and 4B comprising fine powder pigments are formed in stripes to perpendicularly intersect with the surface discharge electrode sets 2H. Generally, the color filter layers 4R, 4G, and 4B are formed from selected materials having optical characteristics such that luminescent colors of the phosphor layers 9R, 9G, and 9B faced to the color filter layers 4R, 4G, and 4B are exclusively allowed to pass through the color filter layers 4R, 4G, and 4B, respectively. Furthermore, the color filter layers 4R, 4G, and 4B are coated with a transparent dielectric layer 5. The transparent dielectric layer 5 has a current limiting function specific to the AC type PDP. The current limiting function will hereinafter be explained. When two adjacent ones of the surface discharge electrode sets 2H are applied with the voltage, surface discharge is caused therebetween. As a result of the discharge, electric charges are stored in the transparent dielectric layer 5. When the sum of the voltage between the surface discharge electrode sets 2H and the voltage owing to the electric charges stored in the transparent dielectric layer 5 becomes smaller than a discharge maintaining voltage, the discharge is stopped.
In order to assure the dielectric strength and to facilitate the production, the transparent dielectric layer 5 is typically formed by preparing a paste mainly containing a low-melting-point glass, applying the paste by thick-film printing, and baking the paste at a high temperature not lower than a softening point of the glass so that the glass is subjected to reflowing. The transparent dielectric layer 5 thus obtained is flat and does not contain air bubbles. The transparent dielectric layer 5 has a thickness on the order between 20 and 40 microns.
Next, a protection layer 6 is formed to cover an entire surface of the transparent dielectric layer 5. The protection layer 6 comprises a MgO thin film formed by vapor deposition or sputtering or a Mgo film formed by printing or spraying. The protection layer 6 has a thickness on the order between 0.5 and 1 micron. The protection layer 6 serves to lower the discharge voltage and to prevent surface sputtering.
On the other hand, a rear substrate 10 is provided with a plurality of data electrodes 8 arranged in stripes to write display data. in FIG. 2, the data electrodes 8 extend in a direction parallel to the drawing sheet. The data electrodes 8 intersect with the surface discharge electrode sets 2H formed on the front substrate 1. As illustrated in FIG. 1, a plurality of barrier ribs 7 are formed typically by thick-film printing so as not to overlap the data electrodes 8 and to extend in parallel to the data electrodes 8. The barrier ribs 7 serve to avoid discharge error and optical crosstalk between neighboring discharge cells 11. The barrier ribs 7 are not illustrated in FIG. 2,
Furthermore, the phosphor layers 9R, 9G, and 9B corresponding to the luminescent colors of red, green, and blue, respectively, are formed by applying three kinds of phosphors in three successive steps, one step for one color, to cover side walls of the barrier ribs 7 and the data electrodes 8. Since the phosphor layers 9R, 9G, and 9B are also formed on the side walls of the barrier ribs 7, phosphor coated areas are increased to achieve high luminance. The formation of the phosphor layers 9R, 9G, and 9B is typically carried out by screen printing.
Thereafter, the front substrate 1 and the rear substrate 10 are coupled face to face to each other with the barrier ribs 7 interposed therebetween in the manner such that the surface discharge electrode sets 2H and the data electrodes 8 perpendicularly intersect with each other. Then, an assembly of the front and the rear substrates 1 and 10 is sealed airtight. A dischargeable gas, such as a mixed gas of He, Ne, and Xe, is confined within the discharge cells 11 at a pressure on the order of 500 Torr.
In each discharge cell 11, a pair of the surface discharge electrode sets 2H are arranged each of which comprises one transparent electrode 2 and one bus electrode 3. In a gap between the surface discharge electrode sets 2H in each pair, the surface discharge occurs to produce plasma in each discharge cell 11. At this time, ultraviolet ray is produced to excite the phosphor layers 9R, 9G, and 9B so that the visible lights of red, green, and blue are produced therefrom Through the color filter layers 4R, 4G, and 4B formed on the front substrate 1, the visible lights are observed as display lights.
As described above, the surface discharge occurs between each pair of the surface discharge electrode sets 2H adjacent to each other. Herein, one and the other of the electrode sets 2H in each pair serve as a scanning electrode and a maintaining electrode, respectively. While the color PDP is actually driven, maintaining pulses are applied between the scanning electrode and the maintaining electrode. In order to cause writing discharge, an electric voltage is applied between the scanning electrode and the data electrode 8 to trigger opposed discharge. By the maintaining pulses subsequently applied, maintaining discharge is generated between the surface discharge electrode sets 2H.
Referring to FIGS. 4 and 5, a reflection AC type opposed discharge color PDP comprises a transparent glass plate as a front substrate 1 with a plurality of X electrodes 12 arranged in stripes. In FIG. 5, the X electrodes 12 extend in a direction perpendicular to the drawing sheet. On the other hand, a rear substrate 10 is provided with a plurality of Y electrodes 15 arranged in stripes.
Referring to FIG. 5, the Y electrodes 15 extend in a direction parallel to the drawing sheet. The X electrodes 12 and the Y electrodes 15 are covered by dielectric layers 5 and 14, respectively, to form capacitors characterizing the AC type color PDP. An AC pulse voltage of several tens to several hundreds kilohertz (kHz) is applied between the X electrodes 12 and the Y electrodes 15 to cause discharge which triggers a display operation. The condensers formed by the X electrodes 12, the Y electrodes 15, and the dielectric layers 5 and 14 have a function similar to the transparent dielectric layer 5 of the surface discharge type described above.
To produce the reflection AC opposed discharge color PDP, the X electrodes 12 are at first formed on the front substrate 1. The X electrodes 12 must be thin so as not to intercept visible lights emitted from phosphor layers 9R, 9G, and 9B. However, when the X electrodes 12 are thin, the resistance is increased. It is therefore required to use metal electrodes having a low resistance. Taking the above into account, the X electrodes 12 are formed by a multilayer thin film of chromium/copper/chromium, a metal thin film such as an aluminum thin film, or a metal thick film using a silver paste.
Next, black masks 13 are formed. In FIG. 4, the black masks 13 are formed to be perpendicular to the drawing sheet and to extend between the X electrodes 12 in parallel to the X electrodes 12. The black masks 13 are formed on the front substrate 1 in order to avoid the decrease in contrast due to white body colors of barrier ribs 7 and the phosphor layers 9R, 9G, and 9B formed on the rear substrate 10. The black masks 13 are formed by direct patterning according to thick-film printing. Alternatively, a photosensitive paste is applied on the front substrate 1 in a solid unpatterned manner and thereafter patterned via exposure and development.
Between the black masks 13, color filter layers 4R, 4G, and 4B are formed in stripes. Generally, the color filter layers 4R, 4G, and 4B are formed from selected materials having optical characteristics such that luminescent colors of the phosphor layers 9R, 9G, and 9B faced to the color filter layers 4R, 4G, and 4B are exclusively allowed to pass through the color filter layers 4R, 4G, and 4B, respectively. On the color filter layers 4R, 4G, and 4B, the transparent dielectric layer 5 and a protection layer 6 are sucessively formed. The purpose and the manner of forming these layers are similar to those described in conjunction with the AC type surface discharge color PDP and will not be described any longer.
On the other hand, the Y electrodes 15 are formed on the rear substrate 11 to perpendicularly intersect with the X electrodes 12 formed on the front substrate 1. In FIG. 4, the Y electrodes 15 extend in parallel to the drawing sheet. The Y electrodes 15 are formed in the manner similar to that mentioned in conjunction with the X electrodes 12. The dielectric layer 14 is formed on the Y electrodes 15. Unlike the transparent dielectric layer 5 formed on the front substrate 1, the dielectric layer 14 need not be transparent. Rather, the dielectric layer 14 is preferably white so as to efficiently reflect the visible lights emitted from the phosphor layers 9R, 9G, and 9B towards the front substrate 1. Like the transparent dielectric layer 5, the dielectric layer 14 is formed by preparing a paste mainly containing a low-melting-point glass, applying the paste by thick-film printing, and baking the paste at a high temperature not lower than a softening point of the glass so that the glass is subjected to reflowing. The dielectric layer 14 thus obtained is flat and does not contain air bubbles. The dielectric layer 14 has a thickness on the order between 15 and 30 microns.
A protection layer 16 is deposited on the dielectric layer 14 as a plurality of protection patterns arranged in stripes and perpendicularly intersecting with the Y electrodes 15. Referring to FIG. 5, the protection layer 16 is perpendicular to the drawing sheet. The protection layer 16 formed on the rear substrate 11 has a function similar to that of the protection layer 6 formed on the front substrate 1. In this opposed discharge type, all discharges, including writing discharge and maintaining discharge, are carried out between the front substrate 1 and the rear substrate 11. It is therefore necessary to form the protection layer 16 on the rear substrate 11 in addition to the protection layer 6 formed on the front substrate 1.
Next, the barrier ribs 7 are formed on the dielectric layer 14 between every adjacent ones of the protection patterns of the protection layer 16. The barrier ribs 7 are formed in stripes to perpendicularly intersect with the Y electrodes 15 and to extend in parallel to the protection patterns of the protection layer 16. In FIG. 4, the barrier ribs 7 are perpendicular to the drawing sheet. In case of the surface discharge color PDP, the discharge occurs between the surface discharge electrode sets 2H (FIG. 2). In contrast, in case of the opposed discharge type in FIG. 4, the discharge occurs between the X electrodes 12 on the front substrate 1 and the Y electrodes 15 on the rear substrate 11. It is noted here that a discharge start voltage and a discharge maintaining voltage widely differ depending upon a discharge gap. Therefore, in case of the surface discharge type, the distance between the transparent electrodes 2 adjacent to each other is very important. On the other hand, in case of the opposed discharge type, the height of the barrier ribs 7 is important. Therefore, the barrier ribs 7 are formed by multilayer thick-film printing or sandblasting.
A discharge cell 17 is defined by every two adjacent ones of the barrier ribs 7, the front substrate 1, and the rear substrate 11. In the discharge cells 17, the phosphor layers 9R, 9G, and 9B corresponding to luminescent colors of red, green, and blue, respectively, are formed by applying three kinds of phosphors in three successive steps, one step for one color. In order to increase the phosphor coated areas so as to achieve high luminance, the phosphor layers 9R, 9G, and 9B are formed also on the side walls of the barrier ribs 7. The phosphor layers 9R, 9G, and 9B are typically formed by screen printing. The phosphor layers 9R, 9G, and 9B must not cover the protection patterns of the protection layer 16 formed between the barrier ribs 7.
Thereafter, the front substrate 1 and the rear substrate 11 are coupled face to face to each other with the barrier ribs 7 interposed therebetween in the manner such that the X electrodes 12 and the Y electrodes 15 perpendicularly intersect with each other. Then, an assembly of the front and the rear substrates 1 and 11 is sealed airtight. A dischargeable gas is confined within the discharge cells 17.
Referring back to FIG. 2, each of the phosphor layers 9R, 9G, and 9B used in the color PDP comprises white powder having very high reflectivity. Thus, the phosphor layers 9R, 9G, and 9B have a white body color. When an external light such as an indoor or outdoor light is incident to the color PDP, the external light is partly absorbed at the upper portion of the barrier ribs and the bus electrodes. Typically, 30% to 50% of the light is reflected. As a result, the contrast is considerably degraded. In order to prevent the reflection of the external light so as to achieve a high-contrast display, it is proposed to cover a panel surface with an ND (Neutral Density) filter having a transmittance of 40 to 80%. In this case, however, the visible lights from the phosphor layers 9R, 9G, and 9B are partly intercepted to decrease the luminance of the color PDP.
In order to suppress the reflection of the external light while minimizing the decrease in luminance, it is proposed to use the color filter layers 4R, 4G, and 4B. Specifically, in correspondence to the luminescent colors of the discharge cells 17 of red, green, and blue, the color filter layers 4R, 4G, and 4B are formed on the front substrate 1 to pass the red light, the green light, and the blue light, respectively. With this structure, it is possible to simultaneously achieve high contrast and high color fidelity.
Generally, the color filter layers 4R, 4G, and 4B comprise fine powder pigments without containing glass frit. For example, the pigments exclusively allowing passage of the red light, the green light, and the blue light, respectively, may comprise following materials.
red: Fe.sub.2 O.sub.3 -based material PA1 green: CoO--Al.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 based material PA1 blue: CoO--Al.sub.2 O.sub.3 based material PA1 Each of these pigments is mixed with resin and a solvent to form a paste. The paste is applied by printing. Thereafter, the solvent is evaporated. After drying, baking is carried out to remove the resin component. Then, on the color filter layers 4R, 4G, and 4B, the transparent dielectric layer 5 are formed by printing, drying, and baking. However, if the color filter layers 4R, 4G, and 4B are formed directly on the surface discharge electrode sets 2H, floating of the bus electrodes 3 occurs to result in open circuits or insufficient dielectric strength when the panel is formed. Such floating of the bus electrodes 3 occurs upon baking of the transparent dielectric layer 5 formed on the color filter layers 4R, 4G, and 4B. The reason is assumed as follows. The bus electrodes 3 formed on the transparent electrodes 2 are weak in bonding force with the transparent electrodes 2. This is because the transparent electrodes 2 are typically formed by depositing tin oxide or ITO according to the thin film technique as described above.
It is assumed that the bus electrodes 3 are formed by the thick film technique. In this event, the bus electrodes 3 after baking have a composition including a glass frit and a conductive metal. The bus electrodes 3 acquire their bonding force from the glass frit softened by baking to be tightly bonded to an underlying layer. However, if the underlying layer includes the transparent electrodes 2 formed by the thin film technique and containing no glass frit, the bonding force of the bus electrodes 3 to the transparent electrodes 2 is weakened even if the glass frit in the bus electrodes 3 is softened by baking.
Furthermore, each of the color filter layers 4R, 4G, and 4B mainly comprises the pigment without containing the glass frit. If the glass frit is mixed with the pigment to form the color filter layer, a light transmission characteristic is degraded, i.e., the luminance is reduced and the color fidelity is deteriorated. Thus, the color filter layers 4R, 4G, and 4B are reduced in performance by half. Taking the above into consideration, it is general that the color filter layers 4R, 4G, and 4B mainly contain the pigments without using the glass frit. When the transparent dielectric layer 5 containing the glass frit is formed on the color filter layers 4R, 4G, and 4B by applying and baking the paste, stress is produced because of difference in thermal expansion among the bus electrodes 3, the color filter layers 4R, 4G, and 4B, and the transparent dielectric layer 5. The stress is concentrated on the bus electrodes 3 weak in bonding force. This results in occurrence of floating of the bus electrodes 3.
As described above, the transparent dielectric layer 5 (the transparent dielectric layer 5 and the dielectric layer 14 in case of the opposed discharge type) has the current limiting (or controlling) function specific to the AC type PDP. The current limiting function greatly depends on the dielectric constant and the thickness of the transparent dielectric layer 5 (the transparent dielectric layer 5 and the dielectric layer 14 in case of the opposed discharge type). In case of the surface discharge type, capacitors are formed by the surface discharge electrode sets 2H and the transparent dielectric layer 5. (In case of the opposed discharge type, capacitors are formed by the X electrodes 12 and the transparent dielectric layer 5 and by the Y electrodes 15 and the dielectric layer 14.) If the color filter layers 4R, 4G, and 4B are formed between the surface discharge electrode sets 2H and the transparent dielectric layer 5 (or between the X electrodes 12 and the transparent dielectric layer 5), electrostatic capacitance is given by a serial combination of the transparent dielectric layer 5 and each of the color filter layers 4R, 4G, and 4B. It is noted here that the color filter layers 4R, 4G, and 4B comprise different materials exclusively allowing passage of the red light, the green light, and the blue light, respectively. As a result, the electrostatic capacitance differs among different colors. This brings about an in increase or a nonuniformity of the opposed discharge voltage.
Furthermore, the transparent electrode 2 in each surface discharge electrode set 2H is formed by the thin film technique such as sputtering and has a thickness between 1000 and 2000 angstroms. On the other hand, the bus electrode 3 has a thickness between 2 and 8 microns. Thus, the electro-static capacitance of the condenser formed by the surface discharge electrode set 2H and the transparent dielectric layer 5 is greatest on the bus electrode 3. When the color filter layers 4R, 4G, and 4B of the different materials corresponding to red, green, and blue are formed on the bus electrode 3, the electrostatic capacitance is different among red, green, and blue cells. This results in an increase or a nonuniformity of the opposed discharge voltage between the scanning electrode and the data electrode 8.
On the other hand, Japanese Unexamined Patent Publication (JP-A) 8-111180 (111180/1996) discloses a DC type color PDP in which each of color filter layers 42a and 42b is smaller in area than a region surrounded by black masks 43, as illustrated in FIG. 6. On a front substrate 41, the color filter layers 42a and 42b are formed except a portion where a cathode 45 is present. Referring to FIG. 6, a reference numeral 44 represents a window. On a rear substrate 46, a display anode 47, a dielectric layer 48, and a phosphor layer 49 are successively formed. Between the black masks 43 and the dielectric layer 48, a plurality of barrier ribs 50 are formed. A display cell 51 is defined as a space surrounded by side walls of adjacent ones of the barrier ribs 50.
With the above-mentioned structure, optimum luminance and optimum contrast can be obtained by narrowing the areas of the color filter layers 42a and 42b.
However, the above-mentioned prior art is related to the DC type color PDP. In case of the DC type color PDP, DC discharge occurs between the cathode 45 and the anode 47. If the color filter layer 42a is formed on the cathode 45, no discharge occurs because the color filter layer 42a is not conductive. AS a result, a display operation can not be carried out. In this connection, the color filter layers 42a and 42b are formed in those regions except a portion where the cathode 45 is present. Consideration will be made about application of this technique to the AC type color PDP. This technique suggests to narrow each of the color filter layers 42a and 42b in area than the region surrounded by the black masks 43 in view of the luminance and the contrast. In case of the AC type color PDP, discharge occurs even if the color filter layer is formed on the electrodes. Thus, no influence is given to the contrast and the luminance even if the color filter layer overlaps the electrodes. Taking into account easiness in production, it is preferred that the color filter layer is also formed on the electrodes.
However, if this PDP is actually produced, the floating of the electrodes occurs as described above to result in open circuits and insufficient dielectric strength. In this event, the PDP can not operate. Even if no open circuit occurs, incoincidence in electrostatic capacitance occurs due to difference in filter material for red, green, and blue. This results in color dependency of the voltage of the opposed discharge occurring between the scanning electrode and the data electrode 8 upon driving the PDP. Consequently, driving is difficult or requires a complicated driving circuit. The above-mentioned prior art does not suggest any approach to solve these problems.
As described above, the AC type color PDP with the color filter layers is disadvantageous. Specifically, if the color filter layers are formed on the surface discharge electrode sets each comprising the transparent electrode and the bus electrode, floating of the bus electrodes occurs, upon baking the transparent dielectric layer formed on the color filter layers, at those portions where the bus electrodes of metal and the color filter layers are brought into contact. This may result in open circuits or insufficient dielectric strength when the PDP is manufactured. The reason is as follows.
The bus electrodes formed on the transparent electrodes are weak in bonding force with the transparent electrodes. In addition, each of the color filter layers mainly contains the pigment without the glass frit. When the transparent dielectric layer containing the glass frit is formed on the color filter layers by applying and baking the paste, thermal expansion differs among the bus electrodes, the color filter layers, and the transparent dielectric layer. In this event, the stress is produced and concentrated on the bus electrodes weak in bonding force. This results in floating of the bus electrodes.
The transparent dielectric layer (or dielectric layer) has the current limiting (or controlling) function specific to the AC type color PDP. This function is achieved by forming the condensers by the surface discharge electrode sets (or the X electrodes) and the transparent dielectric layer or by the Y electrode and the dielectric layer. However, if the color filter layers are formed between the surface discharge electrode sets and the transparent dielectric layer, between the X electrodes and the transparent dielectric layer, or within the transparent dielectric layer, the electrostatic capacitance of the condenser is given by a serial combination of the transparent dielectric layer and each of the color filter layers. However, the color filter layers transparent to the red light, the green light, and the blue light, respectively, are formed by different materials. As a result, the electrostatic capacitance differs among different colors. This brings about an increase or a nonuniformity of the opposed discharge voltage.
Furthermore, the transparent electrode in each surface discharge electrode set has a thickness between 1000 and 2000 angstroms while the bus electrode has a thickness between 2 and 8 microns. Thus, on the bus electrode, the transparent dielectric layer is thinner by the height of the bus electrode than on the transparent electrode. As a result, the portion where the bus electrode exists has a greatest electrostatic capacitance and greatly affects the discharge characterstic of the opposed discharge. Therefore, when the color filter layers are formed between the surface discharge electrode set and the transparent dielectric layer or within the transparent dielectric layer, the electrostatic capacitance is different among different colors. This results in an increase or a nonuniformity of the opposed discharge voltage.